b. Major Program: South Pacific Gyre

The focus at this study site is on life beneath the seafloor in the most oligotrophic region of the world ocean - the South Pacific Gyre (SPG). IODP Expedition 329, led by Co-chief Scientists Steven D’Hondt and Fumio Inagaki, cored and logged deep-sea sediment and basaltic basement at seven SPG sites in 2010. Our present activity in this program focuses on post-expedition studies of samples and data from Expedition 329.

The primary purposes of this project are to:

• Document the nature of microbial communities and test the energetic limit to life in the most food-poor deep-sea sediment,

• Test the influence of basement age and sediment thickness on basement habitability, microbial communities, and the hydrologic evolution of crustal basalt.
This project addresses fundamental questions about subseafloor life, including the following: Is there a lower limit to life in oligotrophic subseafloor sediment? Are the communities in mid-gyre subseafloor sediments uniquely structured (how do these communities compare to those previously studied nearer to the continents)? Is the primary electron donor organic matter from the surface world or hydrogen from in situ radioactive splitting of water? Do microbial activities and composition vary with properties of the surface world, such as sea surface chlorophyll concentrations or organic flux to the seafloor? Is microbial activity sustainable in subseafloor basalt by mineral oxidation (e.g., oxidation of iron in the basaltic minerals) or other processes for tens of Myrs after basalt formation?

Answering these questions will help to achieve all four of the broader C-DEBI themes/objectives. The fundamental goals, activities and outcomes from this reporting period have not different substantially from those originally proposed.

In 2014, we submitted for publication our discovery that microbial cells and organic-fueled aerobic respiration persist throughout the entire SPG sediment sequence (D’Hondt et al., in review). This result indicates that there is no depth limit to microbial life in the most oligotrophic sediment of the open ocean (contrary to the long-standing paradigm of Morita and Zobell, 1955). Our manuscript builds on this discovery to predict that oxygen and aerobic communities may occur throughout the entire sediment sequence in up to 37% of the global ocean. This prediction has major implications for (i) the global nature and distribution of subseafloor life, and (ii) the chemical evolution of Earth’s mantle, volcanic systems and atmosphere. Our model’s primary predictors are sedimentation rate (a principal control on organic flux to the subseafloor sediment) and sediment thickness (the principal control on the timescale of oxygen diffusion through the sediment).

One of the primary 2014 accomplishments of the SPG program was our test of this prediction in a region far from the SPG; the shipboard results of our long-coring expedition KN223 provide strong evidence to confirm our model for subseafloor sedimentary oxygen distribution in the North Atlantic. Our post-expedition studies of KN223 samples and data will test our model’s prediction of community occurrence in deep subseafloor sediment of the North Atlantic.

The SPG program had several additional scientific accomplishments in this project year. These include (i) experimental quantification of radiolytic H₂ production in seawater and marine sediment (Sauvage et al., 2014), (ii) demonstration that the most slowly accumulating and oligotrophic SPG sediment is a net sink of dissolved phosphate from the ocean, whereas more organic-rich sediment at the edge of the SPG and outside it is a net source of dissolved phosphate (Mok et al., in prep.), (iii) demonstration that abundant bacterial taxa (97% similar 16S tags) in anoxic subseafloor sediment are commonly present as rare taxa in the overlying ocean (Walsh et al., 2014), and (iv) calculation that most energy flux to subseafloor sedimentary anaerobes may be used for building biomolecules from existing components (e.g., amino acids in the surrounding sediment), rather than for de novo biosynthesis from inorganic chemicals (D’Hondt et al., 2014).

The first result showed that radiolytic H₂ yield in seawater is the same as in distilled water (contrary to some previous studies) but significantly enhanced in wet zeolite-rich “abyssal clay”. In consequence, radiolytic H₂ may be an especially important electron donor for microbes in organic-poor abyssal clay. The second result has fundamental implications for the global marine phosphorus cycle. The third result implies that subseafloor sedimentary communities are seeded from the overlying ocean during initial sediment deposition. The fourth result has fundamental implications for subseafloor bioenergetics.
Technical Accomplishments

The primary technical advance of the SPG program in 2014 was our experimental quantification of hydrogen yields by gamma radiation in seawater and in natural marine sediment types (Sauvage et al., 2014).

Our primary technical challenges during this reporting period have been development of appropriate techniques for quantitative experimental gamma irradiation and quantitative experimental alpha irradiation of natural wet sediment samples. We solved the problem for gamma irradiation (and undertook key gamma irradiation experiments) (Sauvage et al., 2014). We have designed a solution for quantitative experimental alpha irradiation of natural samples and expect to implement it in the next review period.

► See References Cited in Appendix A

► See related C-DEBI Contributed Publications in Appendix I